The nature and severity of hearing loss among hearing impaired individuals varies widely. Some individuals with linear impairments, such as that resulting from conductive hearing loss, can benefit from the linear amplification provided by conventional hearing aids using analog signal processing. Such aids may have the capacity for limited spectral shaping of the amplified signal using fixed low pass or high pass filters to compensate for broad classes of spectrally related hearing deficits. However, many types of hearing loss, particularly those resulting from inner ear problems, can result in non-linear changes in an individual's auditory system. Individuals who suffer such problems may experience limited dynamic range such that the difference between the threshold hearing level and the discomfort level is relatively small. Individuals with loudness recruitment perceive a relatively small change in the intensity of sound above threshold as a relatively large change in the apparent loudness of the signal. In addition, the hearing loss of such individuals at some frequencies may be much greater than the loss at other frequencies and the spectral characteristics of this type of hearing loss can differ significantly from individual to individual.
Conventional hearing aids which provide pure linear amplification inevitably amplify the ambient noise as well as the desired signal, such as speech or music, and thus do not improve the signal to noise ratio. The amplification may worsen the signal to noise ratio where an individual's hearing has limited dynamic range because the noise will be amplified above the threshold level while the desired speech signal may have to be clipped or compressed to keep the signal within the most comfortable hearing range of the individual.
Although hearing impaired individuals often have unique and widely varying hearing problems, present hearing aids are limited in their ability to match the characteristics of the aid to the hearing deficit of the individual. Moreover, even if an aid is relatively well matched to an individual's hearing deficit under certain conditions, such as a low noise environment where speech is the desired signal, the aid may perform poorly in other environments such as one in which there is high ambient noise level or relatively high signal intensity level.